Hf and Nd Isotopic Constraints on Pre- and Syn-collisional Crustal Thickness of Southern Tibet

In Southern Tibet, voluminous granitoids emplaced between 225-20 Ma provide a spatiotemporal window into the geochemical and tectonic evolution of the crust. Hf and O isotope geochemistry of whole rocks and constituent zircons together with whole-rock chemistry reveal a coherent magmatic history of Gangdese granitoids, and by extension, crustal thickening history of S. Tibet. We observe a spatial isotopic gradient with N-S distance from the Indus-Tsangpo Suture (ITS), with younger, more epsilon(Hf)-positive granitoids adjacent the ITS. Zircons range from epsilon(Hf) = -13 to +11 in a broadly systematic fashion from north to south, generally independent of Pb-206/U-238 age. Adjacent to the ITS, syncollisional (<50 Ma) rocks have generally more heterogeneous eHf than precollisional (>70 Ma) and early syncollisional (50-70 Ma) granitoids, likely reflecting increased assimilation of crustal material in syncollisional magmas as the crust thickened. Zircon delta O-18 ranges between +4 and +8 parts per thousand; syncollisional samples have exclusively mantle-like values (+5.5 to +6 parts per thousand), with greater heterogeneity in precollisional samples. Zircon and whole-rock eHf data reported here are consistent with previous Nd-based thermoisotopic models indicating that the Lhasa block maintained a wedge-shaped crustal geometry from the early Jurassic until the onset of collision. Given evidence of minimal post-50 Ma upper-crustal shortening, these results support earlier findings that the Tibetan crust reached its present similar to 75 km thickness via a roughly equal mixture of upper plate accretion and juvenile magmatic inflation on top of the similar to 30 km-thick of Indian crust underthrust beneath the Lhasa block.